AP Syllabus focus:
‘Earth’s climate has changed throughout geologic time; CO2 records and ice cores show major temperature shifts, including past warming and cooling periods.’
Earth’s climate is not static. Scientists reconstruct past temperature and atmospheric CO2 using natural archives that preserve chemical and physical clues, revealing repeated warming and cooling across geologic time.
What “evidence through geologic time” means
Climate science relies on two overlapping kinds of evidence:
Direct measurements (recent): thermometer and instrumental records (too short for “geologic time”)
Proxy evidence (deep time): indirect indicators stored in Earth materials, used to infer temperature, CO2, and ice volume
A key idea is that multiple independent proxies can be compared; when they tell consistent stories, confidence in past climate patterns increases.
Paleoclimate proxy: A preserved natural record (e.g., ice, sediments) that can be measured to infer past climate conditions such as temperature and atmospheric composition.
Ice cores: a primary archive of CO2 and temperature
Ice cores drilled from Greenland and Antarctica provide some of the clearest, high-resolution records of climate over long timescales.
How ice cores record atmospheric CO2
As snow accumulates and compresses into ice, it traps tiny air bubbles.
Close-up photograph of air bubbles trapped inside glacial ice. These sealed bubbles contain samples of ancient air, allowing scientists to measure past atmospheric greenhouse-gas concentrations (including CO2) directly from ice cores. The scale bar highlights that the bubbles are millimeter-sized features preserved within the ice matrix. Source
Those bubbles preserve samples of the ancient atmosphere, allowing scientists to measure past concentrations of gases such as CO2.
Key features that make ice cores powerful:
Layering: annual or seasonal layers can often be counted, supporting dating
Gas preservation: bubbles act as time capsules for atmospheric composition
Long records: deep cores extend back hundreds of thousands of years (and in some places, longer)
How ice cores record temperature change
Ice cores also contain water molecules with different isotopes (forms of the same element). The relative abundance of heavy vs. light isotopes in the ice varies with temperature at the time the snow fell, enabling reconstruction of past temperature patterns.
In AP Environmental Science terms, the essential takeaway is that ice cores simultaneously track CO2 and temperature, making it possible to compare how both changed during major climate shifts.
What ice cores show about geologic-time climate shifts
Ice-core evidence demonstrates:
Vostok ice-core graph showing atmospheric CO2 (upper curve, ppm) alongside an Antarctic temperature proxy derived from deuterium isotopes (lower curve) over the last ~160,000 years. The paired curves rise and fall together through glacial and interglacial periods, illustrating the close coupling between greenhouse-gas concentrations and temperature in paleoclimate records. This is a core example of how multiple measurements from ice cores can be compared across time. Source
Repeated warming and cooling periods
Large swings between glacial (cold) and interglacial (warm) conditions
Strong association between CO2 records and temperature shifts across these cycles
CO2 records beyond ice cores
While ice cores are central, CO2 records can also be reconstructed using other geologic indicators, which help extend understanding further back in time or cross-check ice-core results.
Geologic and biological indicators that inform past CO2
Common approaches include:
Field photograph of a marine sediment core recovered from the seafloor and prepared for analysis. Layered sediments can preserve microfossils (e.g., foraminifera) and geochemical signals that act as proxies for past ocean conditions and, indirectly, atmospheric CO2. This complements ice-core evidence by extending and cross-checking climate reconstructions using independent archives. Source
Marine sediments: chemical signatures and microfossil composition reflect ocean conditions that connect to atmospheric CO2
Carbonate chemistry in rocks: aspects of carbonate formation can preserve information linked to the carbon cycle and atmospheric conditions
Plant evidence (e.g., fossil leaves): some plant traits correlate with atmospheric CO2 and can be measured in fossils
These records are especially valuable because they:
Provide additional lines of evidence where ice-core coverage is limited
Help confirm that climate has varied substantially across different geologic eras
Interpreting “major temperature shifts” in the record
The specification emphasises that evidence (especially CO2 records and ice cores) shows major temperature shifts, including both warming and cooling. Interpreting those shifts involves comparing patterns across time.
Recognisable patterns scientists look for
Magnitude: how large the temperature change was (small fluctuations vs. major swings)
Rate: how quickly change occurred (gradual transitions vs. rapid shifts)
Duration: how long warm or cold conditions persisted
Consistency across proxies: whether different archives show the same timing and direction of change
Why multiple archives matter
No single proxy is perfect. Strong reconstructions come from agreement among:
Ice-core gas measurements (direct snapshots of past CO2)
Ice-core temperature indicators (isotopic signals)
Independent CO2 indicators from sediments, rocks, and fossils
When these align, it supports the central claim: Earth’s climate has changed throughout geologic time, with repeated, substantial warming and cooling.
Limits and uncertainties (what to remember for APES)
Evidence is robust, but students should understand why reconstructions include uncertainty.
Important limitations include:
Dating uncertainty: deeper layers are harder to date precisely
Resolution changes with age: older records are often less detailed in time
Regional bias: an Antarctic core best represents high-latitude Southern Hemisphere conditions; other records help provide broader context
Proxy interpretation: proxies require calibration and assumptions about how modern relationships applied in the past
Despite these limits, the combined evidence from ice cores and CO2 records clearly documents large climate variability over long timescales, including repeated warming and cooling periods.
FAQ
They combine multiple methods, such as annual layer counting near the surface and matching deeper layers to known time markers.
Common markers include:
volcanic ash layers
abrupt chemistry changes tied to well-dated events
Deeper ice has larger dating uncertainty.
Ice can preserve near-annual to decadal layers in some sections, capturing short-term variability.
Many sediment records are mixed by organisms or currents, so signals can be averaged over longer time intervals, reducing detail.
Different proxies respond to different parts of the carbon cycle and may reflect local conditions.
Disagreement can also come from:
calibration choices
diagenesis (chemical alteration after burial)
time-averaging within sediments
Cross-checking multiple proxies helps resolve this.
Ice cores provide exceptionally direct atmospheric samples but are limited by the age of preserved ice.
Other proxies (sediments, carbonates, fossil leaves) can extend much further back, though they are generally more indirect and may have larger uncertainties.
Variability refers to fluctuations around a baseline across different timescales.
A trend is a persistent directional change over a long interval.
In proxy records, both can appear together, so interpretation depends on timescale and the resolution of the archive.
Practice Questions
State two types of evidence that show Earth’s climate has changed through geologic time. (2 marks)
Identifies ice cores as evidence (1)
Identifies CO2 records as evidence (1)
Describe how ice cores provide evidence for past climate change and explain how they demonstrate major warming and cooling periods through geologic time. (6 marks)
Describes trapped air bubbles in ice preserving ancient atmosphere (1)
Links bubbles to measurement of past CO2 concentrations (1)
Describes that ice (water) isotopes in the core can be used to infer past temperature (1)
States that ice cores contain layered/dated sequences enabling changes to be tracked over time (1)
Explains that the record shows repeated major temperature shifts (glacial/interglacial warming and cooling) (1)
Links the CO2 record and temperature shifts as seen together in ice-core data (association/covariation) (1)
